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#2351
Easy Algorithms

First letter to appear twice

Hash Table String Bit Manipulation Counting
74.9% acceptance
Feb 25, 2026
1182
65
Given a string s consisting of lowercase English letters, return the first letter to appear twice. Note: A letter a appears twice before another letter b if the second occurrence of a is before the second occurrence of b. s will contain at least one letter that appears twice.

Solution

Rust
Time O(n)
Space O(1)
LeetCode
solution.rs 
impl Solution {
  pub fn repeated_character(s: String) -> char {
    let mut seen = 0u32;
    for c in s.chars() {
      let bit = 1u32 << (c as u8 - b'a');
      if seen & bit != 0 { return c; }
      seen |= bit;
    }
    unreachable!()
  }
}